Yes! You can edit or delete all past workouts. Follow these steps:

First, click the side menu and select "Your Workouts". Here you can see workouts, organized by week. Next to each workout is a red "edit" icon. Click that.

After you've clicked this, you'll get to the "Edit Workout" screen. Here you can edit different parts of the workout by clicking the red "Edit Icons" (for example, the Workout Type or the Quantity or the Photo) or you can delete the workout outright, by clicking "Delete Workout" at bottom. If you make edits, make sure to click "Finish Editing" after.